The present invention relates generally to CMOS integrated circuit techniques. More specifically, embodiments of the present invention provide methods and circuits for protecting power amplifier output circuit.
Amplifier circuits are ubiquitous in modern electronic devices. An electronic amplifier increases the power and/or amplitude of a signal. In many applications, power amplifier circuits are used at the output stage of a system to drive an external device. Merely as an example, in an audio system, an output power amplifier is often used to drive an external speaker.
Power amplifier circuits output stages can be classified as class A, B, AB, and C for analog signal amplification. This classification is based on the portion of the input signal cycle during which the amplifying device conducts.
A Class A amplifier operates over the whole of the input cycle such that the output signal is a magnified replica of the whole input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. In a Class A circuit, the amplifying device operated over the linear portion of its characteristic curve. Because the device is always conducting, even if there is no input at all, power is drawn from the power supply. Accordingly, class A amplifiers tend to be relatively in efficient, especially for large power devices.
In contrast, Class B amplifiers only amplify half of the input signal cycle. As such they tend to create signal distortion, but their efficiency is greatly improved over Class A amplifiers. This is because the amplifying element is switched off and does not dissipate power half of the time. An application using Class B amplifiers is the complementary pair or “push-pull” arrangement. Here, complementary devices are used to each amplify the opposite halves of the input signal. The amplified two halves are then recombined at the output. This arrangement gives improved efficiency, but can suffer from the drawback of mismatch at the “joins” between the two halves of the signal, also known as the crossover distortion. An improvement can be achieved by biasing the devices such that neither of the two devices is completely off when they're not in use. This mode of circuit operation is called Class AB operation.
In Class AB operation, each device operates over half the wave similar to Class B operation, but each also conducts over a small signal range in the other half. As a result, when the waveforms from the two devices are combined, the crossover is reduced. Here the two active elements conduct more than half of the time as a means to reduce the cross-over distortions of Class B amplifiers.
In certain applications, it may be desirable to use Class C amplifiers, which conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies are possible. An application for Class C amplifiers is in RF transmitters.
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to drive loudspeakers. Audio signals generally have frequencies between 20 hertz to 20,000 hertz, which is the human range of hearing. In a typical audio system, the audio amplifier is usually preceded by low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and cassette players. Audio systems are used in public address systems, theatrical and concert sound reinforcement, and home sound systems, etc. The sound card in a personal computer often contains several audio amplifiers, as does every stereo or home-theatre system. Audio amplifiers often need to meet stringent performance requirement. In some applications, the input signal to an audio amplifier may measure only a few hundred microwatts. However, its output power may be tens or hundreds of watts.
Because of these requirements, Class AB push-pull circuits are a popular design choice in audio power amplifiers. During much of the time the music is quiet enough that the signal stays in the Class A region, where it is amplified with good fidelity. For large signals, the crossover distortion is much smaller than in the Class AB region, as discussed above. For improved performance, the crossover distortion can be further reduced by using negative feedback techniques.
Even though audio amplifier circuits are widely used in many applications, certain limitations still exist. Some examples are discussed below. FIG. 1A is a simplified view diagram illustrating an output portion 100 of a conventional audio system. As shown in FIG. 1A, an audio frequency signal 102 entering an amplifier 104, which amplifies the signal and drives a speaker 108. A schematic diagram of 100 is shown in FIG. 1B, where the amplifier is shown as a preamplifier 105 and a CMOS output driver circuit 106 including a PMOS driver device and an NMOS driver device. The speaker 108 is shown as an equivalent 8 ohm resistance load.
In this specific example, the class AB amplifier is used for driving the speaker 108. During normal operation, the class AB amplifier delivers an large output current to the small ohmic speaker load, as shown in FIG. 2A. Under certain conditions, e.g., a short circuit at the output node, a large current can flow from the output driver through a wire to the ground, as depicted in FIG. 2B. In audio applications, during normal operation the gate of PMOS transistor P1 can have a voltage range from 0V to Vcc, e.g., 3.3 V. The 0V at the gate causes P1 to be in a hard turn-on condition. If the output node is shorted to ground, the current can be very large. This large current flow can cause damages to the audio system. For example, such large current can sometimes cause melting of bond wires.
Conventional output protection techniques are often ineffective or expensive. For example, extra current monitoring circuits can be used to sense how much current an output amplifier drives to a load. If the current exceeds a certain amount, the monitoring circuit turns the amplifier off. However, this approach has several drawbacks, including additional circuitry, more chip area, and power consumption. Alternatively, the voltage of the output node can be restricted to a safe range. However, this method can lead to degradation of system performance.
Accordingly, a simple and cost-effective method for protecting the output driver circuit is highly desirable.